Time-resolved resonance Raman and density functional theory study of the deprotonation reaction of the triplet state of para-hydroxybenzophenone in mixed acetonitrile/water solutions

Abstract

Nanosecond time-resolved resonance Raman (ns-TR 3) spectroscopy was employed to investigate the triplet state of para-hydroxybenzophenone (p-hBP) in pure acetonitrile (MeCN) and its subsequent deprotonation reaction in a H 2O/MeCN mixed solution. The TR 3 results reveal that the deprotonation reaction occurred with the p-hBP triplet state as the precursor species after 266 nm photolysis of p-hBP in aH 2O/MeCN mixed solution. The ground-state p-hBP anion species was observed to form about 20 ns after excitation in a 5% H 2O/95% MeCN mixed solvent. Density functional theory (DFT) calculations were done to help obtain information about the structures and vibrational modes of the triplet p-hBP and p-hBP anion species. A reaction scheme is proposed for the photoreaction pathways of p-hBP in pure MeCN and H 2O/MeCN mixed solvents. Comparison of the results here for p-hBP to the related para-hydroxyacetophenone (HA) system indicates that substitution of the methyl group in HA with a phenyl group to form p-hBP significantly alters the chemical reactivity of the triplet state so that both deprotonation and intersystem crossing (ISC) for the triplet state occur much faster in p-hBP than in HA. Copyright © 2008 John Wiley & Sons, Ltd.